Ultra violet light is actually harmful to many bacteria as it disrupts
their cell walls. Just as UV-rays can be harmful to you when you spend
too much time on the beach.
UV-light can be used in low doses to cause mutation in bacterial cells,
in higher doses it is lethal.
In short, UV-light will disrupt cell growth and if in a high enough
exposure will kill the cells.
Bryan E. Wilson B.S. RM(NRM)